* Track collection for version_added.
Validate *all* version numbers in validate-modules.
For tagged version numbers (i.e. version_added), consider source collection to chose validation.
* Make tagging/untagging functions more flexible.
* Tag all versions in doc fragments.
* Tag all deprecation versions issued by code.
* Make Display.deprecated() understand tagged versions.
* Extend validation to enforce tagged version numbers.
* Tag versions in tests.
* Lint and fix test.
* Mention collection name in collection loader's deprecation/removal messages.
* Fix error IDs.
* Handle tagged dates in Display.deprecated().
* Also require that removed_at_date and deprecated_aliases.date are tagged.
* Also automatically tag/untag removed_at_date; fix sanity module removal version check.
* Improve error message when invalid version number is used (like '2.14' in collections).
